West suburban police and other emergency officials swarmed a building in Oakbrook Terrace, but it turned out there was no emergency - at least not a real one.

"The lady called, and she said she definitely heard gunshots. So we responded accordingly," said Chief Wayne Holakovsky, Oakbrook Terrace Police Dept.

At least 100 police officers from 30-plus law enforcement agencies responded to the 911 call. It was made, say police, by a woman calling ComEd's customer service line when a planned fire drill set off the alarms. It was not until the first officers arrived on the scene that they realized a drill was taking place, but at that point procedures had to be followed by ComEd and police.

"First order of business is you respond and evacuate. You want to do everything possible to ensure the safety of the employees first," said Terry Donnelly, ComEd chief operating officer.

"We formed teams of four, went through every floor, every desk, every hallway, elevator to make sure everything was secure, to see if we could locate anything," said Holakovsky.

It took well over two hours to search the 11-floor building. Once the all-clear was given, the 1,400 employees at ComEd's Lincoln Center 2 building near Butterfield and Tower Road were able to return to work. The utility company's COO said the false alarm is a sign of the times.

"It was quite a response, but as events that happened in D.C. would indicate, everybody wants to make sure they're responding with all the necessary forces to deal with any emergency that could have been going on at the time, and that makes a lot of sense," said Donnelly.

Holakovsky said it was part of a planned fire drill and training exercise, and the ComEd representative told the woman he had to go and hung up.

But the customer called police at 10:03 a.m. and reported she had been told by the ComEd representative that shots were fired in the building.

A ComEd spokesman tweeted: "During a planned fire drill, we believe a customer being aided by a rep heard the fire alarm & thought it was an actual emergency. A followup tweet said: "Authorities are on site conducting due diligence. Building was evacuated. No confirmed reports of intruders/injuries."

The response led to the closure of roads around the office complex on Trans Am Plaza Drive off Butterfield Road. The response lit up Twitter traffic with reports of gunshots and the emergency response.